This invention relates generally to integrated circuits and more particularly to anti-fuse structures for field programmable gate arrays, programmable read-only memories (PROMS) and the like.
Field programmable gate arrays include a large number of logic elements, such as AND gates and OR gates, which can be selectively coupled together by means of fuses or anti-fuses to perform user designed functions. An unprogrammed fuse-type gate array is programmed by selectively blowing fuses within the device, while an unprogrammed anti-fuse type gate array is programmed by causing selected anti-fuses to become conductive.
There are many types of PROMS including standard, write-once PROMS, erasable programmable read-only memories (EPROMS), electrically erasable programmable read-only memories (EEPROMS) etc. A PROM usually comprises an array of memory cells arranged in rows and columns which can be programmed to store user data. PROMS are typically fuse-type devices.
Fuses for field programmable gate arrays, PROMS and the like are often made from a titanium-tungsten (TiW) alloy and are usually shaped somewhat like a bow-tie having a narrow, central neck and wide ends. The neck of the fuse is typically about 2 microns wide, while the ends of the fuse are typically about 6 microns wide. When a sufficiently high voltage (usually on the order of 10 volts d.c.) is applied to the fuse, the current flowing through the fuse will cause it to heat-up and will eventually melt the fuse at its neck, thereby "blowing" the fuse.
Anti-fuses include a material which initially has a high resistance but which can be converted into a low resistance material by the application of a programming voltage. For example, amorphous silicon, which has an intrinsic resistivity of approximately 1 mega-ohm-centimeter, can be fashioned into 1 micron wide link vias having a resistance of approximately 1-2 giga-ohms. These link vias can then be melted and recrystallized by the application of a programming voltage in the range of 10-12 volts d.c. to form link vias having a resistance less than 100 ohms. These low resistance vias can couple together logic elements of a field programmable gate array so that the gate array will perform user-desired functions, or can serve as memory cells of a PROM.
Fuses in electronic devices are much more prevalent today than anti-fuses because they are easier to manufacture and have a better record of reliability. This is due, in part, to the way that that anti-fuses have been made. Typically, anti-fuses are formed within holes in an insulating layer by some form of deposition process. Deposition processes tend to be somewhat anisotropic, resulting in uneven deposition of the anti-fuse material within the holes due to a shadowing of the bases of the holes by the holes' sidewalls. This shadowing effect creates imperfections in the anti-fuse material, such as creases in the material known as "cusps". These cusps are weak points in the anti-fuse material and can result in the failure of the anti-fuse structure. Moreover, cusping is a strong function of the size of the hole, resulting in non-linear sealability of the process. Unfortunately, even one bad anti-fuse structure in an integrated circuit device may render the entire device defective.
Another problem encountered in making anti-fuses is that the most common anti-fuse material, amorphous silicon, is relatively delicate. For example, it has been found that if aluminum is permitted to contact amorphous silicon that aluminum atoms will diffuse into the silicon, thereby lowering the resistance of the silicon to the point where an anti-fuse structure might appear to have been programmed even if it had not been.
Despite the difficulty in manufacturing anti-fuse structures and their relative lack of reliability, they do have the very desirable feature of being small in size. For example, a TiW fuse with a 2 micron neck and 6 micron end widths permits approximately 4,000 fuses to be provided on a typical device. In contrast, a 1 or 1.2 micron diameter anti-fuse via permits 80,000-100,000 fuses to be provided on a single device. Therefore, arrays of anti-fuses can provide vastly greater numbers of interconnections and can store much larger amounts of information that devices using conventional fuse technology.